In situ sulfur isotope analysis of sulfide minerals by SIMS: Precision and accuracy, with application to thermometry of ∼3.5 Ga Pilbara cherts

Secondary ion mass spectrometry (SIMS) measurement of sulfur isotope ratios is a potentially powerful technique for in situ studies in many areas of Earth and planetary science. Tests were performed to evaluate the accuracy and precision of sulfur isotope analysis by SIMS in a set of seven well-characterized, isotopically homogeneous natural sulfide standards. The spot-to-spot and grain-to-grain precision for delta S-34 is +/- 0.3 parts per thousand for chalcopyrite and pyrrhotite, and +/- 0.2 parts per thousand. for pyrite (2SD) using a 1.6 nA primary beam that was focused to 10 mu m diameter with a Gaussian-beam density distribution. Likewise, multiple delta S-34 measurements within single grains of sphalerite are within +/- 0.3 parts per thousand. However, between individual sphalerite grains, delta S-34 varies by up to 3.4 parts per thousand and the grain-to-grain precision is poor (+/- 1.7 parts per thousand, n = 20). Measured values of delta S-34 correspond with analysis pit microstructures, ranging from smooth surfaces for grains with high delta S-34 values, to pronounced ripples and terraces in analysis pits from grains featuring low delta S-34 values. Electron backscatter diffraction (EBSD) shows that individual sphalerite grains are single crystals, whereas crystal orientation varies from grain-to-grain. The 3.4 parts per thousand variation in measured delta S-34 between individual grains of sphalerite is attributed to changes in instrumental bias caused by different crystal orientations with respect to the incident primary Cs+ beam. High delta S-34 values in sphalerite correlate to when the Cs+ beam is parallel to the set of directions < uuw >, from [111] to [110], which are preferred directions for channeling and focusing in diamond-centered cubic crystals. Crystal orientation effects on instrumental bias were further detected in galena. However, as a result of the perfect cleavage along (100) crushed chips of galena are typically cube-shaped and likely to be preferentially oriented, thus crystal orientation effects on instrumental bias may be obscured. Test were made to improve the analytical precision of delta S-34 in sphalerite, and the best results were achieved by either reducing the depth of the analysis pits using a Kohler illuminated primary beam, or by lowering the total impact energy from 20 key to 13 keV. The resulting grain-to-grain precision in delta S-34 improves from +/- 1.7 parts per thousand to better than 0.6%. (2SD) in both procedures. With careful use of appropriate analytical conditions, the accuracy of SIMS analysis for delta S-34 approaches +/- 03 parts per thousand (2SD) for chalcopyrite, pyrite and pyrrhotite and +/- 0.6 parts per thousand for sphalerite. Measurements of delta S-34 in sub-20 mu m grains of pyrite and sphalerite in similar to 3.5 Ga cherts from the Pilbara craton, Western Australia show that this analytical technique is suitable for in situ sulfur isotope thermometry with +/- 50 degrees C accuracy in appropriate samples, however, sulfides are not isotopically equilibrated in analyzed samples. (C) 2010 Elsevier B.V. All rights reserved.